1. Field of Invention
The invention relates a peak detector, in particular to a peak detector used in a receiver to establish a threshold voltage in a swift manner.
2. Related Art
In general, in a passive optical network (PON), a plurality of Optical Network Units (ONU) is provided in the equivalent number of offices or households, and is connected to a single Optical Line Terminal (OLT) through passive devices. Data can be transmitted from an optical line terminal to an optical network unit through broadcasting, which is called down-transmission. Further, data can be transmitted from an optical network unit to an optical line terminal in a time division multiplexing (TDM) manner, which is called up-transmission, as shown in FIG. 1
In the data up-transmission, each of the optical network units (ONU) is assigned a time slot, and in this time slot, the respective optical line terminal may transmit any number of data packets, as shown in FIG. 2. In other words, each of the user ends is connected to the light wave channel of the station end in a time division multiplexing manner. In data up-transmission, the receiver at the station end is used to receive the light signals from the various user ends.
However, since the powers of light waves transmitted from various user ends are not quite the same, the intensities of the light wave signals reaching the station end are certainly not a constant value after transmitting through various light wave channels. Therefore, in order to raise the efficiency of such time division multiplexing communications, the receiver at the station end must swiftly establish a threshold voltage relative to the light wave intensity transmitted from the respective user end, and convert it into the digital signals of logic “0” and “1” after comparing the analog voltage of the light wave signal with the threshold voltage.
The peak detector may quickly detect and obtain the peak value of the input signal; therefore, in the passive optical network (PON), a peak detector, during the design of the receiver at the station end, is usually used to generate a threshold voltage, that is used to determine logic “0” or logic “1” of the received light wave signal, as shown in FIG. 3. In application, the inaccuracy of the threshold voltage value could lead to the distortion of the duty-cycle of the output signal of the receiver, which would in turn affect the sensitivity of the receiver. Thus, the design of a peak detector, which is capable of producing an accurate peak value and avoiding overshoot while the input signal is weak, and swiftly producing a peak value while the input signal is strong, is a keynote and essential task in this field.
In this respect, refer to FIG. 4 for a more detailed explanation. FIG. 4 is a circuit diagram of a peak detector according to the prior art. As shown in FIG. 4, the voltage difference of an input voltage Vi and output voltage Vo is amplified through an error amplifier AP. When the input voltage Vi is higher than the output voltage Vo, the error amplifier AP outputs a high voltage to turn on the current switch SW, thus charging the capacitor Ch; Conversely, when the input voltage Vi is lower than the output voltage Vo, the current switch SW is turned off, thus the capacitor Ch maintains its peak voltage. In the above description, the current switch may be a diode D (as shown in FIG. 5) or a transistor M (as shown in FIG. 6).
However, in the framework of the prior art, when an optimal peak detector relative to a strong input signal is designed, in order to accelerate the response time of the peak detector, a larger charging current is required. As such, when the input signal is weak, during the process of charging the capacitor to its peak voltage, the current switch can not be turned off in time to cut off the charging current due to the delay of the switch-controlling signal caused by the limited bandwidth of the error amplifier, which in turn results in excessive charging duration, hereby producing an output signal exceeding the peak value. Thus it is evident that if overshoot is to be avoided, then the signal delay of the error amplifier or the charging current must be reduced.
When the high-gain error amplifier is utilized to reduce the impact of the diode or transistor on the threshold voltage for realizing a correct and precise peak detector, since the product of the gain of an element and its bandwidth has a limited range, as such under the condition of not sacrificing the gain of the error amplifier, the signal delay caused by the limited bandwidth of the error amplifier can not be infinitesimally reduced. In addition, if the charging current is reduced, then when the input current is large, the time required to charge the capacitor to its peak voltage will be affected. Therefore, when the scope of variation of the input signal is large, the voltage overshoot of the peak detector and the capacitor charging duration can not both be optimized simultaneously.